The classic X-ray tubes have a thermionic cathode at one end and a fixed anode at the other end. Electrons emitted from the cathode are accelerated by a high potential and impact the anode thereby producing X-rays. The electron beam, which must be tightly focused to produce a high-definition image, produces extreme heating of the anode target. The power capability of this tube is limited by the conductive cooling of the anode target.
High intensity X-ray sources are in increasing demand for applications such as X-ray Lithography for Producing Integrated Circuits, Computerized Tomography for X-ray Imaging, and for X-ray Diffraction for Analyzing Materials. High intensity X-ray sources can be constructed by impinging a high intensity beam of electrons on an anode, but cooling the anode becomes a significant technical problem. A latter advance was the rotating-target tube in which the target is the surface of a metal disk spinning rapidly on bearings inside the vacuum envelope and driven by the rotor of an electric induction motor whose stator is outside the envelope. The rotating anode spreads the heat over an annular area of the target and provides much higher power for a short operating time, as in medical radiography. The ultimate cooling of the anode is mostly by thermal radiation in the high vacuum, so these tubes are inadequate for heavy duty operation. One has to wait for the massive anode to slowly cool.
U.S. Pat. No. 1,160,177 to Kelley discloses an X-ray which uses an externally applied cooling medium with a fixed anode. Some improvement in distributing the heat from the beam can be achieved by steering the electron beam to different parts of the anode. U.S. Pat. No. 2,229,152 to Walsweer and U.S. Pat. No. 4,336,476 to Holland disclose an anode sealed entirely in the vacuum which rotates in response to the field from coils exterior to the vacuum. The heat from the anode must be conducted through bearings irradiated through the vacuum to an external cap. U.S. Pat. No. 4,128,781 to Flisikowski et al. discloses an X-ray tube having a cathode rotatable relative to an anode. Electrons from a rotating cathode are incident on a stationary anode ring. The X-rays are emitted from different positions in space as a cathode is rotated. For most applications it is important that the X-rays be emitted from a fixed position in space.
U.S. Pat. Application Ser. No. 683,988 filed Dec. 12, 1984 by the inventor of the present invention, a continuation of which is now U.S. Pat. No. 4,788,705, describes methods by which the anode is rotated while the cathode is operationally fixed in space. One method is to have the rotating thermionic cathode emit along the axis of rotation and the electron beam is deflected by a stationary magnetic field to a stationary spot on the rotating anode. In another variation, the cathode is held stationary off axis by hanging on bearings from the rotating envelope and being held stationary by a magnetic or gravitational field.
U.S. Pat. Application Ser. No. 843,960 filed Mar. 25, 1986 by the inventor of the present invention, a continuation of which is now U.S. Pat. No. 4,821,305, describes an X-ray tube having the whole vacuum envelope rotate with the anode. The anode being part of the vacuum envelope, it can be cooled from the outside by liquid or air. The cathode also rotates. It is an axially symmetric band of photocathode surface which is illuminated by a focused, stationary spot of light entering the envelope through an auxiliary symmetric transparent window, part of the vacuum envelope. Photoelectrons from the cathode are focused, as by a stationary magnetic field, onto a small stationary spot through which the anode rotates.
Thus, there are many ways a high power X-ray tube can be designed to dissipate the heat over a large area of anode. However, nearly all involve a rotating seal in the form of a sliding 0-ring seal or a Ferrofluidic seal. These seals cause problems by limiting the rotation speed or life of the tube due to seal failure.